2500kVA Mini-Substation - Underground Fan Station

In 2025, ArmCoil Afrika was appointed to design, engineer, and manufacture a 2500kVA 11kV/415V miniature substation for deployment at our client’s Geita
Underground Fan Station in Tanzania.

The miniature substation was a key element in supporting the mine’s underground ventilation infrastructure, supplying power to critical fan stations responsible for
maintaining airflow and air quality deep within the mine.

The project called for a bespoke, mine-ready power solution that could withstand harsh underground conditions, tight space constraints, and complex logistics
associated with remote mining operations. ArmCoil’s scope included complete design and engineering, mechanical and electrical fabrication, factory acceptance
testing, and shipment preparation for cross-border delivery.

 

Key requirements included:

• Transformer rating: 2500kVA 11kV/415V, ONAN cooled
• Type: Outdoor-type mini-substation suitable for underground installation
• Cooling system: Double-row radiator configuration for high thermal efficiency
• Insulating medium: Natural ester oil, chosen for its biodegradable and fire-resistant properties
• Neutral Earthing Resistor (NER): 27-ohm, oil-immersed, integrated onto the base frame
• Protection and control: MV and LV cubicles with switchgear, RMU, and comprehensive control wiring.
• Pressure relief and safety devices: Allbro PRD Type T50, lockable tap switch handle, oil gauge, thermometer, and relief valves
• Fire suppression readiness: Provision for external connection to mine fire systems
• Custom engineered under base: Reinforced design to support transport via rail and tunnel systems underground
• The substation was required to be fully containerized, allowing for rapid installation with minimal site assembly and reduced commissioning time.

The mine’s technical team required a high-capacity and environmentally responsible substation that could integrate seamlessly into the existing underground electrical network.
Due to the confined nature of underground tunnels, the design had to emphasize compactness, mechanical strength, and ease of mobility without sacrificing electrical performance or safety.

True to its innovation-first approach, ArmCoil accepted the challenge and deployed a multidisciplinary team comprising design engineers, protection specialists, manufacturing
technicians, and quality assurance experts. The team worked in close collaboration with the client’s appointed electrical engineer. ArmCoil’s engineering team undertook an extensive concept-to-final design process, producing multiple revisions as the project evolved. Initial design drawings defined the layout of MV and LV cubicles, radiator positioning, and NER mounting. During subsequent design reviews, ArmCoil’s engineers incorporated several structural and mechanical improvements, which introduced significant refinements to the base and mounting systems.

 

Key Engineering Enhancements:

• Reinforced Base Assembly – The under base was redesigned with outer I-beam assemblies, under base stiffeners, and central reinforcement members, ensuring maximum load distribution during transport and handling.
• Integrated NER Mounting – The NER was mounted directly to the base frame to reduce cable runs, improve electrical performance, and maintain a compact footprint.
• Structural Mobility – Additional lifting lugs, grating, and tie-down points were added to simplify underground hoisting and positioning.
• Cable Management – Revised HV and LV gland plates were introduced to enhance accessibility, cable routing, and future maintenance.
• Thermal Optimization – The radiator design and oil circulation paths were modelled for efficient thermal dissipation in confined and hightemperature
  environments.
• Mechanical Aesthetics and Branding – The final model included the engraved ArmCoil logo, nameplate data, and colour specification in Electric Orange,
  consistent with ArmCoil’s standard for visibility and safety compliance.

Through each iteration, ArmCoil’s internal review teams—spanning engineering, quality control, and project management—ensured the design met IEC and ISO standards while aligning with the mine’s operational constraints.

Fabrication took place at ArmCoil’s manufacturing facility in Johannesburg, operating under an ISO 9001 (Quality), ISO 14001 (Environmental), and ISO 45001 (Health & Safety) management system.

All components were manufactured, assembled, and tested in-house to maintain strict control over quality and lead time.

Manufacturing Scope:

• Tank and radiator fabrication, shot blasting, and powder coating
• Precision welding and assembly of the skid base and I-beam structure
• Integration of the MV and LV cubicles, NER, RMU, and auxiliary systems
• Installation of gauges, relief devices, and fittings according to the approved general arrangement
• Oil filling and processing under vacuum to ensure dielectric purity
• Electrical wiring, continuity testing, and final terminal labelling
• Routine, type, and pressure testing in accordance with IEC 60076

A comprehensive Factory Acceptance Test (FAT) was conducted, attended by client representatives.

Testing covered insulation resistance, winding ratio verification, oil dielectric strength, control wiring continuity, and interlock functionality. All results were documented, with minor punch list items rectified before dispatch.

Due to the remote nature of the site, logistics required careful planning.
The completed unit was securely packaged for cross-border transport and fitted with clearly marked lifting points and handling instructions.

ArmCoil’s logistics team coordinated export documentation, customs clearance, and delivery to the Tanzanian border. Upon arrival, the mine’s operations team managed the final leg to site, where the mini substation was installed adjacent to the underground fan control zone.

• A fully containerized and transport-ready substation capable of withstanding rugged underground movemen
• Improved electrical reliability for fan station operations
• Reduced environmental risk through ester oil usage
• Optimized installation efficiency via integrated NER and accessible gland plate configurations
• Enhanced safety through pressure relief, temperature monitoring, and fire suppression interfaces
• The installation achieved successful energization and had since operated within design parameters without any reported failures or abnormal readings.

This project exemplifies ArmCoil’s ability to deliver custom-engineered electrical infrastructure for high demand industrial environments.

The success of the mini-substation demonstrates the synergy between design innovation, manufacturing excellence, and practical field understanding — qualities that continue to define ArmCoil’s reputation in the power and transformer industry.

By combining robust engineering, precision fabrication, and client-driven adaptability, ArmCoil reaffirmed its standing as a trusted partner in providing turnkey power solutions across Africa’s mining sector.